Added a Properties dockwidget

Displays properties of selected objects, tiles and layers at the moment.
Does not display other stuff like size, height, color, etc. yet. My plan
is to do that using the Qt Property Browser solution.
Added the Qt Property Browser solution

A planned replacement for the current primitive property editing table.
Introduced a PropertyBrowser for in the Properties dockwidget

The PropertyBrowser class extends QtTreePropertyBrowser and allows
editing of custom properties as well as built-in properties like layer
names, object size/position, etc.

There is currently no way yet to add/remove custom properties.

The plan is to allow the Properties dockwidget to replace the Properties
dialog.
Set the tree widget as focus proxy of the tree property browser

Makes the focus go to the tree widget when set on the property browser,
since it's not very helpful for it to stay on the property browser
widget.
WIP: Open property dock rather than the dialog